Introduction to respirationRespiration is the process by which cells obtain energy in the form of adenosine tri-phosphate or ATP.

ATP transfers chemical energy from the energy rich substances in the cell to the cell's energy requiring reactions.

When ATP breaks down, the energy created is used by the cell for processes such as active transport, DNAreplication and muscle contraction.

The main respiratory substrate used by cells is 6-carbon glucose.

Respiration is a series of reactions in which 6-carbon glucose is oxidised to form carbon dioxide. The energy released due to the oxidation of glucose is used to synthesize ATP from adenosine diphosphate or ADP and inorganic phosphate or Pi.

Fats and proteins can also be used as respiratory substrates.

Respiration is a stepwise series of reactions involving oxidation and reduction. The mnemonic OIL RIG may help you remember.

In oxidation, energy is released:

  • Oxidation
  • Is
  • Loss; the removal of hydrogen.
In reduction, energy is used up:

  • Reduction
  • Is
  • Gain; the addition of hydrogen.
Breaking bonds releases energy, making bonds requires energy. This is true whether it is in a substrate of respiration or the energy transfer molecule ATP.

Stages of aerobic respiration
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The structure of a mitochondrion
The structure of a mitochondrion

The mitochondria are the site of aerobic respiration.

This has three stages:

  • glycolysis
  • Krebs cycle
  • cytochrome system
Glycolysis
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This stage takes place in the cytoplasm; it does not require oxygen:

  • Glycolysis is the breakdown of 6-carbon glucose into two 3-carbon pyruvic acid units.
  • The hydrogens removed join with the hydrogen carrier NAD to form NADH2.
  • Although some energy is needed to start glycolysis there is an overall net gain of 2 ATP.
  • The pyruvic acid (3C) enters the matrix of the mitochondrion where it is oxidized (i.e. 2H removed) and a carbon dioxide is lost. Thus forming a two carbon molecule called acetyl-CoA (2C).
  • The hydrogens which have been removed join with NAD to form NADH2.


Krebs cycle: citric acid cycle; tricarboxylic acid cycle
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Krebs cycle: citric acid cycle; tricarboxylic acid cycle
This stage takes place in the matrix of the mitochondrion and is the aerobic phase and requires oxygen:

  • It begins when the 2-carbon acetyl CoA joins with a 4-carbon compound to form a 6- carbon compound called Citric acid.
  • Citric acid (6C) is gradually converted back to the 4-carbon compound ready to start the cycle once more.
  • The carbons removed are released as CO2.
  • The hydrogens, which are removed, join with NAD to form NADH2.


Cytochrome system: hydrogen carrier system; electron transfer system
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Cytochrome system: hydrogen carrier system; electron transfer system
Most of the energy produced during respiration is made by the cytochrome system. In this stage of aerobic respiration, the NADH2molecules produced during glycolysis and the Krebs cycle transfer the hydrogens to the cytochrome system.

This is a system of hydrogen carriers located on the cristae of the mitochondrion and this stage also requires oxygen, which acts as the final hydrogen acceptor:

  • The oxygen and the hydrogen combine to form metabolic or respiratory water.
  • If oxygen is not present to act as the final hydrogen acceptor, the hydrogen cannot pass through the system and complete oxidation cannot take place


Anaerobic respirationWhen anaerobic respiration occurs there is no oxygen to act as the final hydrogen acceptor and so the hydrogen cannot pass through the cytochrome system. As a result, both the Krebs cycle and the cytochrome system stages cannot take place:

  • The only ATP produced is formed during glycolysis, that is, 2 ATP.
  • The pyruvic acid produced following glycolysis undergoes a different anaerobic pathway depending on the organism.
In plants: pyruvic acid Ethanol (2C) + Carbon Dioxide (CO2)

In animals: pyruvic acid Lactic Acid (3C)

The pathway in animals is reversible with lactic acid being converted back to pyruvic acid when oxygen is present.

Anaerobic respiration produces only 2ATP compared to the 38 molecules of ATP produced during aerobic respiration.